He was dressed in a suit

1. The verb "dress" (when relating to clothes) can be transitive and intransitive.

(a) When it is a transitive verb 及物動詞, it means "to put clothes on yourself or someone else" (給…穿衣).

Format: to dress somebody (in something)
Example:
His wife dressed him in a suit. (active voice)
He was dressed in a suit. (passive voice) <=== your sentence

(b) When it is intransitive verb 不及物動詞, it means "to put on clothes" (穿上衣服).

Format: to dress (in something)
Example:
I dressed quickly.
She always dressed entirely in black.


2. The verb "wear" (when relating to clothing/decoration) can only be transitive.
In other word, it must take an object and it is, therefore, not necessary to put a preposition "in" after it.

Format: to wear something 穿〔衣服﹑鞋子〕﹐戴〔首飾〕
Example:
She wore a dark blue suit.
She was wearing a new coat.

2014-06-14 02:42:51 補充：
Thank you, Garlic2010. You're right. Normally, it should be viewed as adjective with linking verb "be' and "get", and it'd better use "by" for passive voice.

e.g. The kids were dressed in their best clothes by their mother.

2014-06-14 02:43:24 補充：
Yet, it's confusing. The following was cited from Oxford Learner's dictionary under "dress" as verb:

dress somebody (for/in/as something)
e.g. He was dressed as a woman (= he was wearing women's clothes).

2014-06-14 03:05:14 補充：
As per 網友 "garlic2010" 補充:

"dressed" can also be a predicative adjective linked with "be" and "get" etc.

When it means "wearing clothes of a particular type", it is used together with a preposition "in", such as:

He was dressed in a suit.
He was casually dressed in jeans and a T-shirt.

He was in a brown jacket. Is it correct?

He was dressed in a suit.
'dressed' is an adjective. The sentence is not in passive voice.
e.g.
He was casually dressed in jeans and a T-shirt.
Ref: http://www.oxfordlearnersdictionaries.com/definition/english/dressed

2014-06-16 01:25:05 補充：
Yes, 'dress somebody' is a correct usage but 'he was dressed in a suit' is unambiguously not passive.

"in a suit" 是一個狀態。
指穿著套裝的狀態。